I expect my opinion will be controversial . . .
Based on a recent visit, while the food at Bellagio was much more varied and ambitious than other buffets we tried, I thought it was an overall loser. First, it's exorbitantly expensive ($31.95, as I recall, for a weekend night). Second, the line took over an hour and a half - nothing is worth that kind of wait. Third, the staff were disorganized, rude and ill trained. Fourth, the room was not that nice - it reminded me too much of a windowless Luby's cafeteria from my childhood. Fifth, the very ambition of the food was its downfall - not much of it, in my view, was very successful in the volume it was being prepared and served.
On the other hand, I really LIKED the Flamingo buffet, on a cost/value scale. At about half the cost of Bellagio for dinner, the fare was simpler, but better. On comparable "signature" buffet items, the king crab legs and shrimp, Flamingo had a leg up on Bellagio - they were more flavorful and less watery. Service was immeasurably better. The wait was much, much shorter - about 10 minutes. The room was prettier - instead of a warehouse feel, you're in a tiered multi-area space with a floor to ceiling window fronting a waterfall and fishpond.